Writer Douglas Adams says space is mind-bogglingly big. Here is perhaps an interesting facet for thinking on this topic. Imagine sitting at one end of a room in which at an appointed time a friend enters from the far side. Now imagine that you look and look but cannot see this friend when they first enter, no matter how hard you squint your eyes.
After a while, the person does appear to walk into the room, but the information is delayed from the time at which it actually happened. We are not familiar with such experiences. It does not happen to us on Earth only because of the small size of our planet, but it does happen in outer space.
The reason for this ‘optical illusion’ has to do with the properties of light. Our eyes know that a friend is present only when light from a light bulb, or sunlight coming in through a window, bounces off of that friend and then reflects back into your eyes. On Earth this is not instant, but instead takes some tiny tiny (but non-zero) interval of time amounting to less than a billionth of a second.
This time interval is not so tiny when we consider now moving the setting of the story into space. One doesn’t actually have to go very far away to experience significant apparent ‘disappearances’ of friends.
Imagine this time that a friend is in a space ship in orbit about Jupiter. When the friend appears from the back side of this planet, the light reaches our eyes (or rather our telescopes), about 30 minutes after the friend actually appears! And the effect is mutual.
When the friend circles Jupiter and is in a position to see you send up a gigantic flare into the sky from the Earth, it would be about 30 minutes after the fact before your friend would see the flare.
The farther away, the stronger the effect. From the vantage point of the nearest star, Proxima Centauri, for example, it would take about four years for your friend to see the flare you set off.
From the nearest large spiral galaxy to us, the Andromeda Galaxy it would take about 2.2 million years for the friend (or the friend’s endlessly great grandchildren) to see the flare.
Likewise, if you saw a flare set off by a being in the Andromeda Galaxy all you would know is that some intelligent creature made this deliberate signal 2.2 million years ago. One would not know even if the intelligent species still existed...! And there are 100 billion galaxies.